1. Field of the Invention
The present invention relates to an image forming apparatus such as a color laser printer and a color digital copying machine.
2. Description of the Related Art
In a conventional image forming apparatus, such, as a copying machine, the characteristics of photo conductive body change under the influence of environmental changes, secular changes, and, the like, and an image density may thus be varied. In other words, the densities of copied images may differ from one another even when the same original is copied. It is important for a multi-tone printer, a digital copying machine, or an analog copying, machine to prevent the image density from varying by stabilizing the image density. Particularly in a color copying machine, a variation in image density affects not only density reproducibility, but also the reproducibility of images. It is therefore essential to stabilize the image density.
In the conventional image forming apparatus, a control tolerance for environmental changes and secular changes is applied to a material for constituting the apparatus itself and a process of forming an image so that the image is stabilized by the maintenance of the apparatus. In this apparatus, however, there is a limitation in the control tolerance of the material and process. Maintenance requires labor and costs, and a cycle in which the image density varies is shorter than the maintenance cycle, wherein a stable image density cannot be obtained only by the maintenance.
To control a variation in image density due to environmental and other changes by a cycle shorter than the maintenance cycle, an image forming apparatus has recently been proposed in which the amount of developing agent which is deposited on an image carrier is measured by a measuring device such as a photosensor, and image density is controlled by the result of the measurement. In this image forming apparatus, the measured amount is compared with a predetermined reference value, and image forming conditions such, as an amount of charges applied to the image carrier, the development bias voltage, the amount of exposure and the density of developer are all controlled in accordance with the compared result. The amount of developing agent deposited on the image carrier is thus made constant, and the image density can always be stabilized.
In the above image forming apparatus, the measuring device is placed near the surface of the image carrier on the downstream side of a developing section with respect to the rotating direction of the image carrier. After development is performed in the developing section by supplying a developing agent to a latent image on the image carrier, the measuring device emits light onto the image carrier and detects light reflected by the surface of the image carrier on which the developer is deposited; therefore an amount of developing agent deposited to the image carrier is measured by the measuring device.
Since, however, the measurement surface of the measuring device faces the image carrier, the developing agent flying away from the image carrier may be attached to the measurement surface of the measuring device. If the developing agent is attached to the measurement surface, the measurement precision of the measuring device is lowered, and developer amount of the developer cannot be correctly measured. Consequently, the image density cannot be correctly controlled or constantly stabilized.